In 3rd Generation Partnership Project (3GPP), management of base-station Coordinated Multi-Point (CoMP) transmission in LTE-Advanced that results from extending Long Term Evolution (LTE) has been studied. In the CoMP transmission, multiple base stations (which are also referred to as Base Stations (BS's) or evolved Node B's (eNB's)) transmit data to one terminal (which is also referred to as a Mobile Station (MS) or User Equipment (UE)) in a coordinated manner, and the terminal improves received quality in the terminal itself by performing compositing on pieces of data that are transmitted from the multiple base stations.
FIG. 1 is a diagram illustrating an example of a constitution of a communication system that performs the CoMP transmission, and illustrating an example of the terminal that is present in a cell that is covered by multiple base stations and each of the multiple base stations.
In the following description, a terminal that corresponds to an LTE system (for example, 3GPP Releases 8 and 9) is referred to as an “LTE terminal”, and a terminal that corresponds to an LTE-Advanced system (for example, 3GPP Release 10 or later) is referred to as an “LTE-A terminal”.
As illustrated in FIG. 1, in a case where the CoMP transmission is performed, multiple base stations are constituted from one base station (which is hereinafter also referred to as a connection-making base station or a Master BS) that decides contents of transmission control relating to the CoMP transmission, and a different base station (hereinafter also referred to as a coordination-providing base station or a Slave BS) that performs the CoMP transmission according to an instruction for CoMP transmission control from the connection-making base station.
Each of the connection-making base station and the coordination-providing base station transmits a Channel State Information Reference Signal (CSI-RS) toward the LTE-A terminal. When receiving the CSI-RS that is transmitted from each of the connection-making base station and the coordination-providing base station, the LTE-A terminal measures Channel State Information (CSI) and transmits the measured CSI to the connection-making base station for reporting. That is, the connection-making base station acquires the CSI between the connection-making base station and the LTE-A terminal and the CSI between the coordination-providing base station and the LTE-A terminal from the LTE-A terminal that has a connection to the connection-making base station itself.
Included in the CSI are a Channel Quality Indicator (CQI) indicating channel quality, a Precoding Matrix Indicator (PMI) indicating channel phase information, and the like.
The connection-making base station optimizes control of the CoMP transmission using the CSI for the connection-making base station and the CSI for the coordination-providing base station, which are reported from the LTE-A terminal. Examples of a method of controlling the CoMP transmission, Adaptive Modulation Coding (AMC) control, frequency scheduling, beam control, and the like, are given.
Incidentally, as illustrated in FIG. 1, in some cases, in a communication system, in addition to the LTE-A terminal that is capable of performing the CoMP communication, the LTE terminal is present in a mixed manner. However, as is the case with the CoMP transmission, it is not stipulated that, in the LTE system, a signal is transmitted in a coordinated manner from multiple base station to one LTE terminal. Furthermore, it is stipulated that, in the LTE system, the LTE terminal reports the CSI between the base station and the LTE terminal using a Cell-specific Reference Signal (CRS) that is transmitted from one base station that has a connection. However, it is not stipulated that the LTE terminal reports the CSI for a base station other than the connection-making base station, for example, the coordination-providing base station in FIG. 1, using the CSI-RS that is transmitted from multiple base stations. More precisely, it is difficult to cause the CoMP transmission to apply to the LTE terminal.
In contrast, PTL 1 discloses a method in which the terminal transmits the CQI for the connection-making base station and the CQI for the coordination-providing base station in a time-division manner in every subframe. The base station accomplishes optimization of the CoMP transmission by optimizing transit power of each base station using the CQI for each base station, which is transmitted from the terminal.